toolchain/Config.in: update ucontext handling

As suggested by Thomas Petazzoni in [1], add a comment on
BR2_TOOLCHAIN_HAS_UCONTEXT to specify that this boolean will be set to
true only when a toolchain provides a full featured ucontext
implementation with ucontext_t and {get,make,set}context. As a result,
drop its selection from BR2_TOOLCHAIN_USES_MUSL to fix the following
musl build failure on php:

/home/autobuild/autobuild/instance-8/output-1/host/lib/gcc/m68k-buildroot-linux-musl/12.3.0/../../../../m68k-buildroot-linux-musl/bin/ld: Zend/zend_fibers.o: in function `zend_fiber_init_context':
zend_fibers.c:(.text+0x946): undefined reference to `getcontext'

[1]: https://patchwork.ozlabs.org/project/buildroot/patch/20230516193307.1543455-1-bernd.kuhls@t-online.de/

Fixes:
 - http://autobuild.buildroot.org/results/271f9fb8bfa5ba2f74feef81e6b375b54e21cece

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>

toolchain/Config.in

@@ -34,7 +34,6 @@ config BR2_TOOLCHAIN_USES_MUSL
 	select BR2_TOOLCHAIN_HAS_THREADS
 	select BR2_TOOLCHAIN_HAS_THREADS_DEBUG
 	select BR2_TOOLCHAIN_HAS_THREADS_NPTL
-	select BR2_TOOLCHAIN_HAS_UCONTEXT
 	select BR2_TOOLCHAIN_SUPPORTS_PIE if !BR2_STATIC_LIBS
 
 choice
@@ -302,6 +301,8 @@ config BR2_TOOLCHAIN_HAS_SSP_STRONG
 	bool
 	default y if BR2_TOOLCHAIN_HAS_SSP && BR2_TOOLCHAIN_GCC_AT_LEAST_4_9
 
+# This boolean is true if the toolchain provides a full featured
+# ucontext implementation with ucontext_t and {get,make,set}context
 config BR2_TOOLCHAIN_HAS_UCONTEXT
 	bool