depends: hard-code necessary c(xx)flags rather than setting them per-host

The per-host flag variables hold platform-specific defaults that are ignored
when flag environment variables are set, so it was wrong for them to contain
-std options relied on by package definitions.

Additionally, these flags (-pipe and -std=xx) will no longer be passed into
the CMake build, meaning less duplication in the build summary.

Pulled out of #31920.

depends/hosts/mingw32.mk

@@ -5,8 +5,8 @@ ifneq ($(shell $(SHELL) $(.SHELLFLAGS) "command -v $(host)-g++-posix"),)
 mingw32_CXX := $(host)-g++-posix
 endif
 
-mingw32_CFLAGS=-pipe -std=$(C_STANDARD)
-mingw32_CXXFLAGS=-pipe -std=$(CXX_STANDARD)
+mingw32_CFLAGS=
+mingw32_CXXFLAGS=
 
 ifneq ($(LTO),)
 mingw32_AR = $(host_toolchain)gcc-ar