Makefile: Make $(LLVM) more flexible

Introduce a way for developers to easily switch between LLVM versions
with LLVM=/path/to/llvm/ and LLVM=-version. This is a useful
addition to the existing LLVM=1 variable which will select the first
clang and llvm binutils available on the path.

Reviewed-by: Anup Patel <[email protected]>
Tested-by: Anup Patel <[email protected]>
Signed-off-by: Charlie Jenkins <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Anup Patel <[email protected]>

Author: charlie-rivos

Makefile

@@ -104,10 +104,16 @@ endif
 
 # Setup compilation commands
 ifneq ($(LLVM),)
-CC		=	clang
-AR		=	llvm-ar
-LD		=	ld.lld
-OBJCOPY		=	llvm-objcopy
+ifneq ($(filter %/,$(LLVM)),)
+LLVM_PREFIX := $(LLVM)
+else ifneq ($(filter -%,$(LLVM)),)
+LLVM_SUFFIX := $(LLVM)
+endif
+
+CC		=	$(LLVM_PREFIX)clang$(LLVM_SUFFIX)
+AR		=	$(LLVM_PREFIX)llvm-ar$(LLVM_SUFFIX)
+LD		=	$(LLVM_PREFIX)ld.lld$(LLVM_SUFFIX)
+OBJCOPY		=	$(LLVM_PREFIX)llvm-objcopy$(LLVM_SUFFIX)
 else
 ifdef CROSS_COMPILE
 CC		=	$(CROSS_COMPILE)gcc

README.md

@@ -252,6 +252,18 @@ option with:
 make LLVM=1
 ```
 
+To build with a specific version of LLVM, a path to a directory containing the
+LLVM tools can be provided:
+```
+make LLVM=/path/to/llvm/
+```
+
+If you have versioned llvm tools you would like to use, such as `clang-17`, the LLVM variable can
+be set as:
+```
+make LLVM=-17
+```
+
 When using Clang, *CROSS_COMPILE* often does not need to be defined unless
 using GNU binutils with prefixed binary names. *PLATFORM_RISCV_XLEN* will be
 used to infer a default triple to pass to Clang, so if *PLATFORM_RISCV_XLEN*