[LLD][MinGW] Accept joined format for --stack

Postgresql uses `--stack=` in its Makefile.

Downstream issue: msys2/MINGW-packages#9167

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D107237

(cherry picked from commit 05b025e)

Author: mati865

lld/MinGW/Options.td

@@ -85,7 +85,7 @@ defm output_def: Eq<"output-def", "Output def file">;
 defm section_alignment: Eq<"section-alignment", "Set section alignment">;
 def shared: F<"shared">, HelpText<"Build a shared object">;
 defm subs: Eq<"subsystem", "Specify subsystem">;
-def stack: S<"stack">;
+defm stack: Eq<"stack", "Set size of the initial stack">;
 def strip_all: F<"strip-all">,
     HelpText<"Omit all symbol information from the output binary">;
 def strip_debug: F<"strip-debug">,

lld/test/MinGW/driver.test

@@ -88,6 +88,7 @@ SUBSYSTEM_WINDOWS: -subsystem:windows,7.8
 
 RUN: ld.lld -### foo.o -m i386pep -stack 4194304,8192 2>&1 | FileCheck -check-prefix=STACK %s
 RUN: ld.lld -### foo.o -m i386pep --stack 4194304,8192 2>&1 | FileCheck -check-prefix=STACK %s
+RUN: ld.lld -### foo.o -m i386pep --stack=4194304,8192 2>&1 | FileCheck -check-prefix=STACK %s
 STACK: -stack:4194304,8192
 
 RUN: ld.lld -### foo.o -m i386pep -verbose 2>&1 | FileCheck -check-prefix=VERBOSE %s