[Driver] Explicitly request platform rtlib in the Driver pic test

[phosek]

Summary

This test checks whether crtbegin.o and crtend.o appear on the link
line, but names of these files may be affected by the choice of the
rtlib, specifically when compiler-rt is used as the default rtlib
the names will be clang_rt.crtbegin.o and clang_rt.crtend.o instead
of crtbeginS.o and crtendS.o. To avoid the test failure, explicitly
request to use the platform rtlib.

clang/test/Driver/pic.c

 // RUN: %clang -c %s -target i386-unknown-unknown -fpic -fPIE -fpie -### 2>&1 \
 // RUN:   | FileCheck %s --check-prefix=CHECK-PIE1
 // RUN: %clang -c %s -target i386-unknown-unknown -fpie -fPIC -fPIE -### 2>&1 \
 // RUN:   | FileCheck %s --check-prefix=CHECK-PIE2
 //
 // Make sure -pie is passed to along to ld and that the right *crt* files
 // are linked in.
 // RUN: %clang %s -target i386-unknown-freebsd -fPIE -pie -### \
-// RUN: --gcc-toolchain="" \
+// RUN: --gcc-toolchain="" -rtlib=platform \
 // RUN: --sysroot=%S/Inputs/basic_freebsd_tree 2>&1 \
 // RUN:   | FileCheck %s --check-prefix=CHECK-PIE-LD
 // RUN: %clang %s -target i386-linux-gnu -fPIE -pie -### \
-// RUN: --gcc-toolchain="" \
+// RUN: --gcc-toolchain="" -rtlib=platform \
 // RUN: --sysroot=%S/Inputs/basic_linux_tree 2>&1 \
 // RUN:   | FileCheck %s --check-prefix=CHECK-PIE-LD
 // RUN: %clang %s -target i386-linux-gnu -fPIC -pie -### \
-// RUN: --gcc-toolchain="" \
+// RUN: --gcc-toolchain="" -rtlib=platform \
 // RUN: --sysroot=%S/Inputs/basic_linux_tree 2>&1 \
 // RUN:   | FileCheck %s --check-prefix=CHECK-PIE-LD
 //
 // Disregard any of the PIC-specific flags if we have a trump-card flag.
 // RUN: %clang -c %s -target i386-unknown-unknown -mkernel -fPIC -### 2>&1 \
 // RUN:   | FileCheck %s --check-prefix=CHECK-NO-PIC
 
 // The -static argument *doesn't* override PIC: -static only affects