[compiler-rt][llvm-lit] Enable internal shell for compiler-rt test suite

[connieyzhu]

Testing in lit is currently inconsistent across different platforms, as many RUN line commands are written in a way that only works on POSIX-compliant shells. Lit has an internal shell implementation to ensure uniform execution of these commands that can be used regardless of platform. We should aim to turn on the internal shell for the compiler-rt test suite to eliminate reliance on external shells.

The internal shell has missing capabilities that need to be addressed before doing so. Here is a list of the issues that block us from turning on the internal shell for compiler-rt.

• [llvm-lit] Environment variable not found in compiler-rt subproject with lit internal shell  #106598
  [llvm-lit] Ulimit command not found in lit internal shell #102398
  [llvm-lit] Unset command not found in lit internal shell #102397
  [llvm-lit] Expected string not found in input error in compiler-rt when testing with lit internal shell #102695
  [llvm-lit] InternalShellError in compiler-rt test using export command #102386
  [llvm-lit] Attribute Error in lit internal shell #102399
  [llvm-lit] Unhashable TypeError 'GlobItem' with lit’s internal shell #102389
  [llvm-lit] lit internal shell failing to parse and execute for-loop syntax #102380
  [llvm-lit] Unsupported redirect in compiler-rt test using lit internal shell #102385
  [llvm-lit] lit internal shell failing to parse and execute command substitution syntax #102384
  [llvm-lit] lit internal shell fail to execute parentheses syntax #102401
  [llvm-lit] lit internal shell failing to parse and execute curly brace syntax #102382
  [llvm-lit][compiler-rt] Test using 'export' failing to execute with lit internal shell due to too many arguments #106142
  [llvm-lit] Expand and Rewrite Tests for FileNotFoundError for compiler-rt tests in lit internal shell #106111