Remove -march=knl flag for GCC 15 compatibility

GCC 15 dropped support for Xeon Phi (KNL) targets, making -march=knl
invalid. For optimized KNL kernels, remove -march=knl since -mavx512f
is already specified explicitly. For reference kernels, replace
-march=knl with -march=skylake to provide an AVX2 baseline (AVX-512
is then disabled via -mno-avx512f -mno-avx512cd).

Follows the same approach as PR 129 which removed -mavx512pf.

Closes gh-150

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rgommers

blis/_src/config/knl/make_defs.mk

@@ -75,13 +75,13 @@ endif
 # Flags specific to optimized kernels.
 CKOPTFLAGS     := $(COPTFLAGS)
 ifeq ($(CC_VENDOR),gcc)
-CKVECFLAGS     := -mavx512f -mfpmath=sse -march=knl
+CKVECFLAGS     := -mavx512f -mfpmath=sse
 else
 ifeq ($(CC_VENDOR),icc)
 CKVECFLAGS     := -xMIC-AVX512
 else
 ifeq ($(CC_VENDOR),clang)
-CKVECFLAGS     := -mavx512f -mfpmath=sse -march=knl
+CKVECFLAGS     := -mavx512f -mfpmath=sse
 else
 $(error gcc, icc, or clang is required for this configuration.)
 endif
@@ -91,21 +91,21 @@ endif
 # The assembler on OS X won't recognize AVX512 without help.
 ifneq ($(CC_VENDOR),icc)
 ifeq ($(OS_NAME),Darwin)
-CKVECFLAGS     += -Wa,-march=knl
+CKVECFLAGS     += -Wa,-march=skylake-avx512
 endif
 endif
 
 # Flags specific to reference kernels.
 # Note: We use AVX2 for reference kernels instead of AVX-512.
 CROPTFLAGS     := $(CKOPTFLAGS)
 ifeq ($(CC_VENDOR),gcc)
-CRVECFLAGS     := -march=knl -mno-avx512f -mno-avx512cd -funsafe-math-optimizations -ffp-contract=fast
+CRVECFLAGS     := -march=skylake -mno-avx512f -mno-avx512cd -funsafe-math-optimizations -ffp-contract=fast
 else
 ifeq ($(CC_VENDOR),icc)
 CRVECFLAGS     := -xMIC-AVX512
 else
 ifeq ($(CC_VENDOR),clang)
-CRVECFLAGS     := -march=knl -mno-avx512f -mno-avx512cd -funsafe-math-optimizations -ffp-contract=fast
+CRVECFLAGS     := -march=skylake -mno-avx512f -mno-avx512cd -funsafe-math-optimizations -ffp-contract=fast
 else
 $(error gcc, icc, or clang is required for this configuration.)
 endif