musl: add missing AArch64 HWCAP_* consts

[tarcieri]

Closes #2171.

I effectively just copied and pasted these from #1638 after confirming that they are identical to musl's aarch64/bits/hwcap.h and that musl's getauxval() also takes a c_ulong.

It seems like if these are added it would address some of TODO-linux.txt?

[rust-highfive]

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[Amanieu]

I think that does address the TODO-linux.txt, you can remove them from this file.

[tarcieri]

@Amanieu done

[Amanieu]

@bors r+

[bors]

📌 Commit 20b2b2c has been approved by Amanieu

[bors]

⌛ Testing commit 20b2b2c with merge 393a80d...

[bors]

💔 Test failed - checks-actions

[tarcieri]

Build failure looks unrelated

[joshtriplett]

Let's give it another try:
@bors r+

[bors]

💡 This pull request was already approved, no need to approve it again.

• This pull request previously failed. You should add more commits to fix the bug, or use retry to trigger a build again.

[bors]

📌 Commit 20b2b2c has been approved by joshtriplett

[bors]

⌛ Testing commit 20b2b2c with merge 4d4a90e...

[joshtriplett]

@bors retry

[bors]

⌛ Testing commit 20b2b2c with merge ae698a1...

[bors]

💔 Test failed - checks-cirrus-freebsd-11

[bors]

☀️ Test successful - checks-actions, checks-cirrus-freebsd-11, checks-cirrus-freebsd-12, checks-cirrus-freebsd-13
Approved by: joshtriplett
Pushing ae698a1 to master...