Rollup merge of #37091 - alexcrichton:configure, r=brson

configure: Fix gcc detection for LLVM

We have a case where 32-bit compilation accidentally requested clang when gcc
was the only one available.

Author: alexcrichton

configure

@@ -1635,8 +1635,8 @@ do
         ("ccache gcc")
             LLVM_CXX_32="ccache"
             LLVM_CC_32="ccache"
-            LLVM_CXX_32_ARG1="clang++"
-            LLVM_CC_32_ARG1="clang"
+            LLVM_CXX_32_ARG1="g++"
+            LLVM_CC_32_ARG1="gcc"
 
             LLVM_CXX_64="ccache"
             LLVM_CC_64="ccache"