kernel-selftest: remove flag assignments only

The do_compile() step removes flags from the kernel Makefile
before building the selftests.

On older kernels (pre-5.12), -mrecord-mcount is guarded by a
cc-option probe:
    ifeq ($(call cc-option-yn,-mrecord-mcount),y)
        CC_FLAGS_FTRACE += -mrecord-mcount
    endif
Previously, the sed expression matched mrecord-mcount regardless
of its context in the line. This removed both the flag assignment
and the ifeq guard, leaving an orphaned endif and causing the
selftest build to fail.

Anchor the expression on += so that the sed only removes flag
assignments, giving us more precise control over which Makefile
lines are deleted.
Apply the same adjustment to the -Wno-alloc-size-larger-than
expressions for consistency.

Signed-off-by: Leonid Iziumtsev <leonid.iziumtsev@est.tech>
Signed-off-by: Khem Raj <khem.raj@oss.qualcomm.com>

meta-oe/recipes-kernel/kernel-selftest/kernel-selftest.bb

@@ -170,9 +170,9 @@ either install it and add it to HOSTTOOLS, or add clang-native from meta-clang t
     fi
     oe_runmake -C ${S} headers
     sed -i -e 's|^all: docs|all:|' ${S}/tools/testing/selftests/bpf/Makefile
-    sed -i -e '/mrecord-mcount/d' ${S}/Makefile
-    sed -i -e '/Wno-alloc-size-larger-than/d' ${S}/Makefile
-    sed -i -e '/Wno-alloc-size-larger-than/d' ${S}/scripts/Makefile.*
+    sed -i -e '/+=[[:space:]]*-mrecord-mcount/d' ${S}/Makefile
+    sed -i -e '/+=[[:space:]]*-Wno-alloc-size-larger-than/d' ${S}/Makefile
+    sed -i -e '/+=[[:space:]]*-Wno-alloc-size-larger-than/d' ${S}/scripts/Makefile.*
 
     # Add kernel headers to CFLAGS to fix PTP selftest compilation
     # Required for PTP_MASK_CLEAR_ALL and PTP_MASK_EN_SINGLE definitions