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/linux.mk

@@ -1,5 +1,5 @@
-linux_CFLAGS=-pipe -std=$(C_STANDARD)
-linux_CXXFLAGS=-pipe -std=$(CXX_STANDARD)
+linux_CFLAGS=
+linux_CXXFLAGS=
 
 ifneq ($(LTO),)
 linux_AR = $(host_toolchain)gcc-ar