Makefile: fix linking on macOS with Homebrew

On macOS with current version of Homebrew clang and ld.lld comes from
separate packages, the assumptions for the linker path does not hold
anymore and the linking fails.

Fix by using distinct variables in Makefile for clang and ld.lld:

- TOOLCHAIN: remove the check for `uname -p` and use `brew --prefix`
  output instead of hardcoded path
- add a LLDDIR variable to allow for specific ld.ldd location
- LLDDIR: use TOOLCHAIN value if ld.lld is found there, use the output
  of `brew --prefix lld` otherwise
- LD: use LLDIR instead of TOOLCHAIN

Fixes: #431

Signed-off-by: Alexis Deruelle <alexis.deruelle@gmail.com>

Makefile

@@ -3,10 +3,12 @@ RUSTARCH ?= aarch64-unknown-none-softfloat
 ifeq ($(shell uname),Darwin)
 USE_CLANG ?= 1
 $(info INFO: Building on Darwin)
-ifeq ($(shell uname -p),arm)
-TOOLCHAIN ?= /opt/homebrew/opt/llvm/bin/
+BREW ?= $(shell command -v brew)
+TOOLCHAIN ?= $(shell $(BREW) --prefix llvm)/bin/
+ifeq ($(shell ls $(TOOLCHAIN)/ld.lld 2>/dev/null),)
+LLDDIR ?= $(shell $(BREW) --prefix lld)/bin/
 else
-TOOLCHAIN ?= /usr/local/opt/llvm/bin/
+LLDDIR ?= $(TOOLCHAIN)
 endif
 $(info INFO: Toolchain path: $(TOOLCHAIN))
 endif
@@ -17,10 +19,14 @@ else
 ARCH ?= aarch64-linux-gnu-
 endif
 
+ifneq ($(TOOLCHAIN),$(LLDDIR))
+$(info INFO: LLD path: $(LLDDIR))
+endif
+
 ifeq ($(USE_CLANG),1)
 CC := $(TOOLCHAIN)clang --target=$(ARCH)
 AS := $(TOOLCHAIN)clang --target=$(ARCH)
-LD := $(TOOLCHAIN)ld.lld
+LD := $(LLDDIR)ld.lld
 OBJCOPY := $(TOOLCHAIN)llvm-objcopy
 CLANG_FORMAT ?= $(TOOLCHAIN)clang-format
 EXTRA_CFLAGS ?=