[cmake] Add LoongArch to config.guess

[xen0n]

Summary

Necessary for native builds on LoongArch. Only Linux is added for now,
matching upstream config.git changes.

Unfortunately, the upstream config.guess is GPLv3+ since 2012, and
LLVM is without autoconf support for a long time, so the script's
licensing exception no longer works and a bump could not be trivially
done.

This is the same as the upstream commit adding LoongArch,
but rewritten to match the 2011 style that our script is holding on
to.

[MaskRay]

LGTM.

[SixWeining]

llvm/cmake/config.guess

925

I'm not sure it exists or not. Let me double check with others internally. Thanks.

926

Unlike the upstream config.git, this file misses the initialization of LIBC. Perhaps we can use gnu for now?

[xen0n]

Fix libc detection; the 2011 script is not initializing $LIBC beforehand.

[xen0n]

llvm/cmake/config.guess

925

IIRC it was planned to be something like the MIPS n32, but due to the later shift to the RISC-V-like model it should no longer exist. The "LPX32" ABI should never see the light of day.

But again here I'm just updating the script to match the upstream behavior. If loongarchx32 should be deleted here, then the upstream config.git should get updated too.

926

I just found out this a few minutes ago (in my test repo I just copied over the upstream script so I didn't notice). Fixed.

[SixWeining]

LGTM. Thanks.

llvm/cmake/config.guess

925

I agree with you. Let's remove it after upstream config.git has done so.

[xen0n]

Note to $random_passer_by_with_commit_access: I don't currently have commit access, so please help me land the patch (and also D130821 which is also accepted) at your leisure. Thanks.

llvm/cmake/config.guess

EOF
   #endif
 EOF
   eval `$CC_FOR_BUILD -E $dummy.c 2>/dev/null | grep '^LIBC'`
   echo "${UNAME_MACHINE}-pc-linux-${LIBC}"
   exit ;;
     ia64:Linux:*:*)
   echo ${UNAME_MACHINE}-unknown-linux-gnu
   exit ;;
+    loongarch32:Linux:*:* | loongarch64:Linux:*:* | loongarchx32:Linux:*:*)
+  LIBC=gnu
+  eval $set_cc_for_build
+  # Do not check for __GLIBC__ because uclibc defines it too
+  sed 's/^ //' << EOF >$dummy.c
+  #include <features.h>
+  #if defined(__UCLIBC__)
+  LIBC=uclibc
+  #elif defined(__dietlibc__)
+  LIBC=dietlibc
+  #endif
+EOF
+  eval `$CC_FOR_BUILD -E $dummy.c 2>/dev/null | grep '^LIBC'`
+
+  # There is no features test macro for musl
+  # Follow the GNU's config.guess approach of
+  # checking the output of ldd
+  if command -v ldd >/dev/null && \
+    ldd --version 2>&1 | grep -q ^musl; then
+      LIBC=musl
+  fi
+
+  echo "${UNAME_MACHINE}-unknown-linux-${LIBC}"
+  exit ;;
     m32r*:Linux:*:*)
   echo ${UNAME_MACHINE}-unknown-linux-gnu
   exit ;;
     m68*:Linux:*:*)
   echo ${UNAME_MACHINE}-unknown-linux-gnu
   exit ;;
     mips:Linux:*:* | mips64:Linux:*:*)
   eval $set_cc_for_build