[roll] Roll llvm-project/libc [libc] Remove _Exit proxy func header and use LIBC_NAMESPACE::_Exit in tests (#114904)

This improves/fixes this pr
llvm/llvm-project#114718. In this PR we removed
the _Exit proxy func because it was not needed. Instead we used
`LIBC_NAMESPACE::_Exit`

GitOrigin-RevId: e71903630a42d9d22f68634f2f833cfbc4c82572
Original-Revision: 7a1d7e7fcd4655b6f2b7efe120e442734a8d20bd
Change-Id: I30d44087e7ed9f28ccbb61b799854bbf2c5a2dbc

Author: Job Henandez Lara

third_party/flower

@@ -372,7 +372,7 @@
              gitsubmoduleof="fuchsia"
              path="third_party/llvm-libc/src"
              remote="https://llvm.googlesource.com/llvm-project/libc"
-             revision="065baebc5b373054aa5908a91c505c649fb70581"/>
+             revision="7a1d7e7fcd4655b6f2b7efe120e442734a8d20bd"/>
     <project name="third_party/lz4"
              gitsubmoduleof="fuchsia"
              path="third_party/lz4"