[gn build] Set llvm_have_mallinfo2 to true by default

[aeubanks]

Summary

If you are using an old enough glibc which doesn't have mallinfo2, set
llvm_have_mallinfo2 = false.

At this point it's likely that most people using the gn build are
compiling against a recent enough glibc (glibc 2.33 which was released
in Feb 2021).

[peterwaller-arm]

Thanks for the note, I appreciate it.

[peterwaller-arm]

For anyone wandering the internet in search of an answer to all their woes (probably future me, Hi future me!), this is the relevant error.

FAILED: obj/llvm/lib/Support/Support.Process.o
clang++ -MMD -MF obj/llvm/lib/Support/Support.Process.o.d -o obj/llvm/lib/Support/Support.Process.o -c ../../llvm/lib/Support/Process.cpp  -I../../llvm/lib/Support/Unix -I../../llvm/lib/Support/Windows -I../../llvm/include -Igen/llvm/include -O3 -fdiagnostics-color -Wall -Wextra -Wno-unused-parameter -Wdelete-non-virtual-dtor -Wstring-conversion -no-canonical-prefixes -Werror=date-time -fPIC -Wcovered-switch-default -std=c++14 -fvisibility-inlines-hidden -fno-exceptions -fno-rtti
In file included from ../../llvm/lib/Support/Process.cpp:107:
../../llvm/lib/Support/Unix/Process.inc:93:20: error: variable has incomplete type 'struct mallinfo2'
  struct mallinfo2 mi;
                   ^
../../llvm/lib/Support/Unix/Process.inc:93:10: note: forward declaration of 'mallinfo2'
  struct mallinfo2 mi;
         ^
../../llvm/lib/Support/Unix/Process.inc:94:10: error: no member named 'mallinfo2' in the global namespace
  mi = ::mallinfo2();
       ~~^
2 errors generated.

llvm/utils/gn/secondary/llvm/include/llvm/Config/BUILD.gn

declare_args() {
   # Enable additional checks that alter the LLVM C++ ABI.
   llvm_enable_abi_breaking_checks = llvm_enable_assertions
 
   # Iterate unordered llvm containers in reverse.
   llvm_enable_reverse_iteration = false
 
   # glibc is at least 2.33 which has mallinfo2.
   # TODO: remove this once nobody using the gn build is building against an old glibc.
-  llvm_have_mallinfo2 = false
+  llvm_have_mallinfo2 = true
 }
 
 write_cmake_config("abi-breaking") {
   input = "abi-breaking.h.cmake"
   output = "$target_gen_dir/abi-breaking.h"
   values = []
 
   if (llvm_enable_abi_breaking_checks) {