For most of the platforms which do not need any machine-specific
stanzas, provide a "default" case and stop enumerating them.

Index: lib/libm/Makefile
===================================================================
RCS file: /OpenBSD/src/lib/libm/Makefile,v
retrieving revision 1.123
diff -u -p -r1.123 Makefile
--- lib/libm/Makefile	21 Jan 2022 03:12:55 -0000	1.123
+++ lib/libm/Makefile	25 Mar 2024 19:42:05 -0000
@@ -47,25 +47,13 @@ ARCH_SRCS = e_sqrt.c e_sqrtf.c e_remaind
 .elif (${MACHINE_ARCH} == "sh")
 .PATH:	${.CURDIR}/arch/sh
 ARCH_SRCS = e_sqrt.c e_sqrtf.c s_fabsf.c
-.elif (${MACHINE_ARCH} == "aarch64")
-.PATH:	${.CURDIR}/arch/aarch64
-.elif (${MACHINE_ARCH} == "arm")
-.PATH:	${.CURDIR}/arch/arm
-.elif (${MACHINE_ARCH} == "m88k")
-.PATH:	${.CURDIR}/arch/m88k
-.elif (${MACHINE_ARCH} == "mips64")
-.PATH:	${.CURDIR}/arch/mips64
 .elif (${MACHINE_ARCH} == "mips64el")
 .PATH:	${.CURDIR}/arch/mips64
-.elif (${MACHINE_ARCH} == "powerpc")
-.PATH:	${.CURDIR}/arch/powerpc
-.elif (${MACHINE_ARCH} == "powerpc64")
-.PATH:	${.CURDIR}/arch/powerpc64
-.elif (${MACHINE_ARCH} == "riscv64")
-.PATH:	${.CURDIR}/arch/riscv64
 .elif (${MACHINE_ARCH} == "sparc64")
 .PATH:	${.CURDIR}/arch/sparc64
 ARCH_SRCS = e_sqrtl.c
+.else
+.PATH:	${.CURDIR}/arch/${MACHINE_ARCH}
 .endif
 
 .PATH:	${.CURDIR}/man