[compiler-rt] [FMV][AArch64] Add initial AT_HWCAP3 / AT_HWCAP4 support (PR #161595)

[Brad Smith via llvm-commits]

================
@@ -190,3 +190,19 @@
 #ifndef HWCAP2_CSSC
 #define HWCAP2_CSSC (1UL << 34)
 #endif
+
+#ifndef AT_HWCAP3
+#if defined(__linux__) || defined(__ANDROID__)
----------------
brad0 wrote:

> AT_HWCAP and AT_HWCAP2 are so old that i assume we don't support compiling with a libc that doesn't have them.

getauxval() was added in glibc 2.16. AArch64 support was added in glibc 2.17. There is some room for simplifying some of this stuff since the expectation is that any system that is on AArch64 has some of these bits included unlike with 32-bit ARM if you go back far enough.

https://github.com/llvm/llvm-project/pull/161595