igvm_c/Makefile: fix path for cross compilation (#92)

When cross-compiling with `CARGO_BUILD_TARGET`, the build output path
changes to include the target triple (e.g., `target/<triple>/release/`).

This leads to compilation errors as `TARGET_PATH` does not take that
into account.

Update `TARGET_PATH` to include the target triple if set.

Signed-off-by: Luigi Leonardi <leonardi@redhat.com>

Author: luigix25

igvm_c/Makefile

@@ -9,9 +9,9 @@ IGVM_DIR := $(API_DIR)/..
 TARGET_DIR ?= target_c
 
 ifdef RELEASE
-TARGET_PATH="$(IGVM_DIR)/$(TARGET_DIR)/release"
+TARGET_PATH="$(IGVM_DIR)/$(TARGET_DIR)/$(CARGO_BUILD_TARGET)/release"
 else
-TARGET_PATH="$(IGVM_DIR)/$(TARGET_DIR)/debug"
+TARGET_PATH="$(IGVM_DIR)/$(TARGET_DIR)/$(CARGO_BUILD_TARGET)/debug"
 endif
 
 PREFIX ?= /usr