[lld] [LLD][ELF][AArch64] Add support for SHF_AARCH64_PURECODE ELF section flag (3/3) (PR #125689)

[Csanád Hajdú via llvm-commits]

================
@@ -2179,6 +2181,34 @@ template <class ELFT> void Writer<ELFT>::checkExecuteOnly() {
                             "data and code";
 }
 
+// Check that all input sections of .text have the SHF_AARCH64_PURECODE section
----------------
Il-Capitano wrote:

My thinking was that anything outside of .text is the responsibility of the toolchain, but I hadn't thought about other use cases you mentioned. Thanks for pointing those out.

> When there's more than one program header with PF_E and PF_R it will be useful to logically group the output per program header.

Can you clarify this a bit? With the current code if I check every `PF_R | PF_X` section I get this output currently from one of my test cases:
```
ld.lld: warning: -z execute-only-report: /lib/aarch64-linux-gnu/Scrt1.o:(.text) does not have SHF_AARCH64_PURECODE flag set
ld.lld: warning: -z execute-only-report: /lib/aarch64-linux-gnu/crti.o:(.text) does not have SHF_AARCH64_PURECODE flag set
ld.lld: warning: -z execute-only-report: /usr/lib/gcc/aarch64-linux-gnu/12/crtbeginS.o:(.text) does not have SHF_AARCH64_PURECODE flag set
ld.lld: warning: -z execute-only-report: /usr/lib/gcc/aarch64-linux-gnu/12/crtendS.o:(.text) does not have SHF_AARCH64_PURECODE flag set
ld.lld: warning: -z execute-only-report: /lib/aarch64-linux-gnu/crtn.o:(.text) does not have SHF_AARCH64_PURECODE flag set
ld.lld: warning: -z execute-only-report: test.lto.o:(.text) does not have SHF_AARCH64_PURECODE flag set
ld.lld: warning: -z execute-only-report: test.lto.o:(.text.b) does not have SHF_AARCH64_PURECODE flag set
ld.lld: warning: -z execute-only-report: /lib/aarch64-linux-gnu/crti.o:(.init) does not have SHF_AARCH64_PURECODE flag set
ld.lld: warning: -z execute-only-report: /lib/aarch64-linux-gnu/crtn.o:(.init) does not have SHF_AARCH64_PURECODE flag set
ld.lld: warning: -z execute-only-report: /lib/aarch64-linux-gnu/crti.o:(.fini) does not have SHF_AARCH64_PURECODE flag set
ld.lld: warning: -z execute-only-report: /lib/aarch64-linux-gnu/crtn.o:(.fini) does not have SHF_AARCH64_PURECODE flag set
ld.lld: warning: -z execute-only-report: <internal>:(.plt) does not have SHF_AARCH64_PURECODE flag set
```
The diagnostics follow each other by their output section, `.text`, `.init`, `.fini`, and `.plt`. (Also, the last warning has `<internal>:(.plt)`, which I'm not sure how to handle yet.)

https://github.com/llvm/llvm-project/pull/125689