LLVM patches from Arm ET 19.1.1 #1
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This test was failing because picolibc did not define TIME_UTC, but more recent versions do now do this.
Previously, building libc for AArch64 in `LLVM_LIBC_FULL_BUILD` mode would fail because no implementation of setjmp/longjmp was available. This was the only obstacle, so now a full AArch64 build of libc is possible. This implementation automatically supports PAC and BTI if compiled with the appropriate options. I would have liked to do the same for MTE stack tagging, but as far as I can see there's currently no predefined macro that allows detection of `-fsanitize=memtag-stack`, so I've left that one as a TODO. AAPCS64 delegates the x18 register to individual platform ABIs, and allows them to choose what it's used for, which may or may not require setjmp and longjmp to save and restore it. To accommodate this, I've introduced a libc configuration option. The default is on, because the only use of x18 I've so far encountered uses it to store information specific to the current stack frame (so longjmp does need to restore it), and this is also safe behavior in the default situation where the platform ABI specifies no use of x18 and it becomes a temporary register (restoring it to its previous value is no worse than any _other_ way for a function call to clobber it). But if a platform ABI needs to use x18 in a way that requires longjmp to leave it alone, they can turn the option off.
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This series contains the LLVM 19.1.1 patches from LLVM Embedded Toolchain for Arm.