[RISCV] Fix sysroot tests without GCC on RISC-V hosts with GCC

D68391 added tests that check scenarios where no RISC-V GCC toolchain is
supposed to be detected. When running the tests on RISC-V hosts the system's
GCC toolchain will be detected, and the tests will fail. This patch adds a
`--gcc-toolchain` option pointing to a path where no GCC toolchain is
present, ensuring that the tests are run under the expected conditions, and
therefore are able to pass in all test environments.

Differential Revision: https://reviews.llvm.org/D75061

(cherry picked from commit 91f7f0d)

Author: luismarques

clang/test/Driver/riscv32-toolchain-extra.c

@@ -19,6 +19,7 @@
 // RUN: ln -s %S/Inputs/basic_riscv32_nogcc_tree/bin/riscv32-unknown-elf-ld %T/testroot-riscv32-baremetal-nogcc/bin/riscv32-unknown-elf-ld
 // RUN: ln -s %S/Inputs/basic_riscv32_nogcc_tree/riscv32-unknown-elf %T/testroot-riscv32-baremetal-nogcc/riscv32-unknown-elf
 // RUN: %T/testroot-riscv32-baremetal-nogcc/bin/clang %s -### -no-canonical-prefixes \
+// RUN:    --gcc-toolchain=%T/testroot-riscv32-baremetal-nogcc/invalid \
 // RUN:    -target riscv32-unknown-elf --rtlib=platform 2>&1 \
 // RUN:    | FileCheck -check-prefix=C-RV32-BAREMETAL-ILP32-NOGCC %s
 

clang/test/Driver/riscv64-toolchain-extra.c

@@ -19,6 +19,7 @@
 // RUN: ln -s %S/Inputs/basic_riscv64_nogcc_tree/bin/riscv64-unknown-elf-ld %T/testroot-riscv64-baremetal-nogcc/bin/riscv64-unknown-elf-ld
 // RUN: ln -s %S/Inputs/basic_riscv64_nogcc_tree/riscv64-unknown-elf %T/testroot-riscv64-baremetal-nogcc/riscv64-unknown-elf
 // RUN: %T/testroot-riscv64-baremetal-nogcc/bin/clang %s -### -no-canonical-prefixes \
+// RUN:    --gcc-toolchain=%T/testroot-riscv64-baremetal-nogcc/invalid \
 // RUN:    -target riscv64-unknown-elf --rtlib=platform 2>&1 \
 // RUN:    | FileCheck -check-prefix=C-RV64-BAREMETAL-LP64-NOGCC %s