Overhaul windows activation scripts

[h-vetinari]

So far we haven't really used these (as flang wasn't widely rolled out), but now that flang 19 looks to be getting in a usable state, we should definitely fix this. This is following inspiration from the clang-win-activation feedstock (taking into account the updates from conda-forge/clang-win-activation-feedstock#40), as well as the config used in the scipy build scripts (as of conda-forge/scipy-feedstock#269)

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.

I do have some suggestions for making it better though...

For recipe/meta.yaml:

• It looks like the 'flang_linux-64' output doesn't have any tests.

[h-vetinari]

Very weird that this started failing.

[281/393] Building CXX object lib/Frontend/CMakeFiles/obj.flangFrontend.dir/FrontendActions.cpp.o
FAILED: lib/Frontend/CMakeFiles/obj.flangFrontend.dir/FrontendActions.cpp.o 
$BUILD_PREFIX/bin/x86_64-conda-linux-gnu-c++ -DFLANG_LITTLE_ENDIAN=1 -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I$SRC_DIR/build/lib/Frontend -I$SRC_DIR/flang/lib/Frontend -I$SRC_DIR/flang/include -I$SRC_DIR/build/include -fvisibility-inlines-hidden -fmessage-length=0 -march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O2 -ffunction-sections -pipe -isystem $PREFIX/include -fdebug-prefix-map=$SRC_DIR=/usr/local/src/conda/flang-split-18.1.8 -fdebug-prefix-map=$PREFIX=/usr/local/src/conda-prefix -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -fno-lifetime-dse -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wno-maybe-uninitialized -Wno-nonnull -Wno-class-memaccess -Wno-redundant-move -Wno-pessimizing-move -Wno-noexcept-type -Wdelete-non-virtual-dtor -Wsuggest-override -Wno-comment -Wno-misleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -Wno-deprecated-copy -Wno-ctad-maybe-unsupported -fno-strict-aliasing -fno-semantic-interposition -O3 -DNDEBUG -fno-semantic-interposition -std=c++17   -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS  -fno-exceptions -funwind-tables -MD -MT lib/Frontend/CMakeFiles/obj.flangFrontend.dir/FrontendActions.cpp.o -MF lib/Frontend/CMakeFiles/obj.flangFrontend.dir/FrontendActions.cpp.o.d -o lib/Frontend/CMakeFiles/obj.flangFrontend.dir/FrontendActions.cpp.o -c $SRC_DIR/flang/lib/Frontend/FrontendActions.cpp
{standard input}: Assembler messages:
{standard input}:246334: Warning: end of file not at end of a line; newline inserted
{standard input}:247285: Error: unbalanced parenthesis in operand 1.
{standard input}: Error: open CFI at the end of file; missing .cfi_endproc directive
x86_64-conda-linux-gnu-c++: fatal error: Killed signal terminated program cc1plus
compilation terminated.

The only real change in the build env that I can see is going from GCC 12.3 -> 12.4; let's see if GCC 13 helps. Actually, it's probably getting OOM-killed because flang is very memory-hungry (on windows we're already on -j1 because of that). Let's try adding a swap file

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.